This invention relates generally to improvements in heating devices, and more particularly, to a heating device utilizing a cartridge-type electric heating unit and a heat transfer member, such as a heat pipe, for heating a liquid in a chamber or storage container.
One presently available widely used electric hot water heater includes a hot water tank and one or two electric heating elements mounted in the wall of the tank. Typically, the heating element is of the so-called hairpin type including a mounting flange and a copper sheathed U-shaped heating element. The element itself is a helical resistance coil which is located within and electrically insulated from a tubular copper sheath. The ends of the tubular sheath are fixed in a mounting flange which is adapted to be secured in a fluid-tight manner to a wall of the tank about an opening in the tank wall with the sheathed heating element disposed within the tank. Electric power is supplied to the heating element through terminals extending outwardly of the flange from insulated ends of the sheathed element.
Such heating elements are constructed with different power, i.e., wattage, ratings, to meet different heating requirements. Also, it is common practice to supply at least four different watt densities (i.e., watt/sq. in. of copper sheath area) for each wattage. Watt density is the main factor which determines the life expectancy and cost of a conventional heating element. High watt density elements (175-300 W/in..sup.2) offer economy in cost, but are prone to build up mineral deposits or scales from the water and high internal operating temperatures, as a result of which such elements are more prone to failure and have a short life expectancy. On the other hand, low watt density elements (40-80 W/in..sup.2) have a longer life expectancy, but are more costly. Both types of elements plus elements having intermediate watt densities are commonly used.
An electric heating element which is, in some cases, more economical than the hairpin type is the so-called cartridge-type heating element. However, a cartridge-type heating element with the same wattage/watt density ratio as a hairpin-type heating element is rather large. In other words, to achieve the necessary sheath area in a tank of small diameter, it is necessary to use large diameter cartridge heating elements (1 inch or more). With such large diameters, the operating temperatures within the element become excessive and shorten the life of the element. As a result, cartridge-type heating elements are not in widespread use in presently available domestic electric hot water heaters.
As will be described more fully hereinafter, the present invention provides an economical heating device which utilizes cartridge-type heating elements. Also, as more fully described hereinafter, this is achieved by utilizing a heat pipe in the heating device.
Heretofore, it has been proposed to utilize a heat pipe in a fossil fuel fired hot water heater as disclosed in U.S. Pat. No. 3,854,454. As will be readily apparent from the following description, the present invention differs in several respects from the heat pipe hot water heater disclosed in this patent. In particular, and as more fully described hereinafter, the present invention has a novel construction utilizing an electric cartridge heating element and one or more heat pipes and provides an improvement over presently available electric hot water heaters and heaters used in hemodialysis equipment